A boy/girl who is good enough to hook up with but you would never make your boyfriend/girlfriend.

-You just hook up and never actually get to know the poor thing. And if it ever wanted to get to know you, you wouldn't give it a chance because it's the underdog.

underdog: that hook up was amazing, wanna date me?
trainer: uh.. no. i just like making out with you.
(the trainer would not date the underdog because for whatever reason the underdog is not bf/gf material. but it is acceptable to make out with it.)

An underdog is something that always seems to be trailing behind, but not always as bad as what’s more popular. Often times when an underdog becomes more know, people new to it tend to cram it down your throat and preach to you how much better it is then everything else. They will also go on Urban Dictionary and write definitions about how great it is, and trash everything else.

Random FireFox User:
OMG everyone says FireFox Ownz, and its the underdog also, so it must be good. Anyone who doesn't have Firefox is a stupid nubzor. All you little nub kids need to get it now! I don't even know why it's good, everyone else told me it was.

When someone (usually a male) hangs upside down while someone else does either one of the following given:
1) Gives you a blowjob
2) Gives you a handjob
3) Fists you. Usually associated with homosexuals.
OR
When a female hangs upside down and you insert your dick in her vagina.

Guy 1," Get on the monkey bars and get into an underdog!"
Girl1 -"Orrr, You can get into an underdog and I can give you pleasure."